"Jonas Salk attributed this finding to changes in the virus that had occurred since the previous year."
Historical Perspective — Emergence of Influenza A (H1N1) Viruses
Zimmer 2009, http://www.nejm.org/doi/full/10.1056/NEJMra0904322

The total influenza vaccine failure of 1947 revisited: Major intrasubtypic antigenic change can explain failure of vaccine in a post-World War II epidemic
Kilbourne 2001, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC125033/